Echoes commonly occur because of imperfect coupling of incoming signals at 4-to-2 wire junctions in communications systems. The echoes typically result because of imperfect impedance matching to the 2-wire facility in the 4-to-2 wire junction causing the incoming signal to be partially reflected over an outgoing path to the source of incoming signals.
Self-adapting echo cancelers have been employed to mitigate the echoes by generating an estimate of the reflected signal or echo and subtracting it from the outgoing signal. The echo estimate is updated in response to the outgoing signal for more closely approximating the echo to be cancelled. Heretofore, the updating of the echo estimate has been inhibited when near end speech signals are being transmitted or when no significant far end energy is being received. However, the echo estimate was allowed to be updated when any significant far end energy was being received, whether it was speech, noise, single frequency tones, multifrequency tones or the like.
It has been determined that allowing the canceler to update the echo estimate during intervals that the received far end signal includes energy occupying only a portion of a frequency band of interest, for example, a single frequency tone, multifrequency tone or the like (hereinafter designated partial band energy), results in an undesirable condition of the communications circuit including the canceler. Specifically, the canceler includes a self-adapting processor which can adjust to a large number of transfer functions in order to generate the echo estimate which best approximates the echo. A problem with allowing the processor to adjust the transfer function when partial band energy is being received is that although the transfer function arrived at is optimized for the frequency components of the partial band energy it may not be optimum for the remaining frequency components in the frequency band of interest, for example, the voice band. Indeed, the transfer function adjusted to at frequencies other than those in the partial band energy may be significantly different from the desired optimum adjustment which would be obtained when adjusting on a whole band signal, i.e., speech or Gaussian noise. Consequently, a so-called low return loss path is established at frequencies other than the partial band energy. This low return loss can lead to oscillations in the communications circuit. These oscillations are extremely undesirable and must be avoided.
The problem of low return loss and other problems of prior echo canceler arrangements resulting from allowing the canceler to adjust the echo estimate during intervals that partial band far end energy is being received are overcome by inhibiting the canceler from adjusting the echo estimate when partial band energy is being received as disclosed in copending U.S. patent application Ser. No. 240,977 filed concurrently herewith. The received energy is discriminated as being partial band or whole band and the echo canceler is enabled to adjust the echo estimate only when whole band energy is being received. That is to say, the echo estimate is enabled to be adjusted during intervals that significant far end energy is being received and the energy is determined not to be partial band and, therefore, is defined as being whole band. The apparatus employed to discriminate between partial band and whole band energy as disclosed in the application Ser. No. 240,977 is sophisticated and presently would require a substantial area of an integrated chip, if not, several chips.